Ring-Geometry Photodetector Designs For High-Sensitivity And High-Speed Detection Of Optical Signals For Fiber Optic And Integrated Optoelectronic Devices

ABSTRACT

A semiconductor photodetector comprising a closed loop configured to receive light from an external source adapted to trap light within said closed loop until absorption by the semiconductor.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.63/128608, filed on Dec. 21, 2020, which is incorporated herein in itsentirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENT

This invention was made with government support by the Office of NavalResearch under Grant No. N00014-17-1-2416. The government has certainrights in the invention.

INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

Not applicable.

FIELD OF THE INVENTION

This invention pertains to the field of photodetectors. Moreparticularly, the invention pertains to novel designs forhigh-sensitivity, high-speed photodetectors.

BACKGROUND OF THE INVENTION

In many applications, including optical telecommunications, an importantcomponent of the system is the photodetector, which converts an opticalsignal to an electrical signal. To provide a fast and sensitivephotodetector, there are several parameters that need to be optimized.Junction capacitance and photocarrier drift time need to be minimizedfor fast response, while absorbance needs to be maximized for highsensitivity. To achieve these two competing goals, photodiode designcompromises are made in current designs.

A typical photodetector is comprised of a junction of p-type and n-typesemiconductors, where the free electrons and empty electron states(holes) combine and create a depletion zone of high electrical field.Photons absorbed in this zone generate free electrons and holes, whichare moved apart rapidly in the electrical field. Once the electrons andholes leave the depletion zone, a current is detected. Photocarrierdrift time describes how long the generated holes (electrons move fasterand are not the limiting factor) take to leave the depletion zone. Thistime can be calculated with the formula of

${t_{h} = \frac{W}{v_{h}}},$

where W is the width of the depletion zone and v_(h) is the holevelocity [Kasap 2013].

Charged dopants in the depletion zone create a high electrical field.The junction capacitance C_(dep) is given by the formula

${C_{dep} = \frac{ɛ_{0}ɛ_{r}A}{W}},$

where A is the area of the photodetector and ε₀ε_(r) is the electricalpermittivity of the material used. Finally, since photons need to beabsorbed in the depletion zone, the absorbance is governed by theequation I(x)=I₀ exp(−αx), where I₀ is the incoming light intensity, αis the absorption coefficient that is dependent on material andwavelength, and x is the distance into the material.

Taken together, a very fast photodetector must have a very thin activeregion (small width of the depletion zone) to minimize drift times. Thethin active region also needs to occupy a very small area to minimizethe capacitance of the junction, which is the other primary limitingfactor on how rapidly the signal can be generated by the photodetector.This extremely small and thin photodetector cannot absorb much of thelight impinging on it perpendicular to the p-n junction plane. Most ofthe incident light will pass through, reducing the detection efficiency.By the same token, a very sensitive photodetector requires either athick depletion zone, which will extend the time for carriers to exitthe depletion zone, or a thin depletion zone that is very large, wherethe capacitance of the junction will limit the speed of detectablesignal.

An alternative design is a long, thin photodetector, with a smalljunction area, called a waveguide photodetector (WGPD). Light in thiscase is impinging along the waveguide axis, in the p-n junction plane.WGPDs can be used to mitigate the problems described above. A bandwidthof over 100 GHz and a quantum efficiency of 50% have been realized [Kato1999]. Even with these extremely fast photodetectors, there are alsoissues with nonlinear absorption of the light down the length of thephotodetector that put limits on dynamic range and degrade performancein digital communication [Williams 1996].

BRIEF SUMMARY OF THE INVENTION

In one embodiment, the present invention provides a semiconductorphotodetector comprising a closed loop configured to receive light froman external source adapted to trap light within said closed loop untilabsorption by the semiconductor.

In other embodiments, the present invention provides semiconductorphotodetector comprising a closed loop configured to receive light froman external source to trap light within the closed loop and recirculatethe light until absorption by the semiconductor.

In other embodiments, the present invention provides semiconductorphotodetector wherein the closed loop recirculates light received froman external source until all light is absorbed by the semiconductor.

In other embodiments, the present invention provides semiconductorphotodetector wherein the closed loop is a ring.

In other embodiments, the present invention provides semiconductorphotodetector wherein light is captured and re-circulated in thephotodetector using a curved ridge-waveguide ring resonator.

In other embodiments, the present invention provides semiconductorphotodetector wherein light is captured and re-circulated in thephotodetector using straight waveguides and mirrors.

In other embodiments, the present invention provides semiconductorphotodetector wherein light is captured and re-circulated in thephotodetector using straight waveguides and mirrors that redirect lightinto the closed loop.

In other embodiments, the present invention provides semiconductorphotodetector wherein light is captured and re-circulated in thephotodetector using a photonic crystal structure.

In other embodiments, the present invention provides a method capturinglight in a semiconductor photodetector comprising the following steps:directing light into a closed loop; and trapping light from an externalsource within said closed loop and recirculating light in the closedloop until absorption by the semiconductor.

Additional objects and advantages of the invention will be set forth inpart in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention will be realized and attained bymeans of the elements and combinations particularly pointed out in theappended claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numeralsmay describe substantially similar components throughout the severalviews. Like numerals having different letter suffixes may representdifferent instances of substantially similar components. The drawingsillustrate generally, by way of example, but not by way of limitation, adetailed description of certain embodiments discussed in the presentdocument.

FIG. 1A shows a waveguide bending photodetector design for an embodimentof the present invention.

FIG. 1B shows a total internal reflection mirrors photodetector designfor an embodiment of the present invention.

FIG. 1C shows a photonic crystal design for an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Detailed embodiments of the present invention are disclosed herein;however, it is to be understood that the disclosed embodiments aremerely exemplary of the invention, which may be embodied in variousforms. Therefore, specific structural and functional details disclosedherein are not to be interpreted as limiting, but merely as arepresentative basis for teaching one skilled in the art to variouslyemploy the present invention in virtually any appropriately detailedmethod, structure or system. Further, the terms and phrases used hereinare not intended to be limiting, but rather to provide an understandabledescription of the invention.

As described above, there are fundamental limits in the physics ofphotodetectors that have forced trade-offs of either sensitivity orfrequency response with current photodetector designs. In one aspect,the embodiments of the present invention concern designs that avoidthese trade-offs by providing a compact, efficient and high-bandwidthring photodetector. Preferred embodiments include three relatedphotodetector designs using the total internal reflection waveguiding inFIG. 1 a, the mirror reflection in FIG. 1 b, or the photonic crystalguiding in FIG. 1 c.

A common feature of the embodiments of the present invention is theentry point for light of that may be a whistle-geometry ringphotodetector (WRP). A ridge waveguide 1 a, 1 b, as shown in FIGS. 1aand 1 b, or a photonic crystal guiding structure 1 c, as shown in FIG. 1c, collects light, either from an optical fiber, lens, or other part ofa larger integrated optoelectronic device and guides it to the ringphotodetector. The light is guided to a non-symmetrical Y-junction,which is shown in FIGS. 1a-1c as parts 2 a, 2 b and 2 c, injecting thelight into the absorptive part of the photodetector. The Y-junctionallows the light to enter the unidirectional sections 3 a, 3 b and 3 cwhich define a continuous path or ring such as a circle, rectangle,hexagonal as well as other designs such as oval, elliptical, and others,of the photodetector, where it can circulate until it is fully absorbed.The path of the light is shown as 4 a, 4 b, and 4 c for the threeembodiments shown in FIGS. 1a -1 c. While the design in FIG. 1are-circulates light by bending the waveguide, FIG. 1b uses strategicallyplaced plurality of mirrors, labeled as 5 b, to induce mirror reflectionfor keeping the light recirculating.

With the ring or continuous path designs of FIG. 1a -c, the absorptivedepletion zone area may be engineered to be very thin in order tominimize drift time, and even though the absorption per unit length islow, the re-circulation of the light within the ring or continuous pathmeans that all of the light will eventually be absorbed. The ring orcontinuous path can be made extremely small (1 μm in diameter or less),minimizing the area of the diode junction, which minimizes both thecapacitance and the physical footprint for an integrated optoelectronicdevice for very large-scale integration (VLSI), where footprint size ofdevices is a limiting factor.

In other embodiments, unidirectional sections 3 a, 3 b and 3 c define aring or continuous path 4 a-4 c which are of a length that causes thelight to re-circulate multiple times while the light is absorbed. Theseembodiments provide much more even illumination for the photodetector,minimizing optical nonlinear effects caused by the extremely small sizesused in the embodiments of the present invention.

While the foregoing written description enables one of ordinary skill tomake and use what is considered presently to be the best mode thereof,those of ordinary skill will understand and appreciate the existence ofvariations, combinations, and equivalents of the specific embodiment,method, and examples herein. The disclosure should therefore not belimited by the above described embodiments, methods, and examples, butby all embodiments and methods within the scope and spirit of thedisclosure.

What is claimed is:
 1. A semiconductor photodetector comprising: aclosed loop, said closed loop configured to receive light from anexternal source; and said closed loop adapted to trap light within saidclosed loop until absorption by the semiconductor.
 2. The semiconductorphotodetector of claim 1 wherein said closed loop recirculates lightreceived from an external source.
 3. The semiconductor photodetector ofclaim 1 wherein said closed loop recirculates light received from anexternal source until all light is absorbed by the semiconductor.
 4. Thesemiconductor photodetector of claim 1 wherein said closed loop is aring
 5. The semiconductor photodetector of claim 1 wherein light iscaptured and re-circulated in the photodetector using a curvedridge-waveguide ring resonator.
 6. The semiconductor photodetector ofclaim 1 wherein light is captured and re-circulated in the photodetectorusing straight waveguides and mirrors.
 7. The semiconductorphotodetector of claim 1 wherein light is captured and re-circulated inthe photodetector using straight waveguides and mirrors, said waveguidesand mirrors redirecting light into said closed loop.
 8. Thesemiconductor photodetector of claim 1 wherein light is captured andre-circulated in the photodetector using a photonic crystal structure.9. A method capturing light in a semiconductor photodetector comprisingthe following steps: directing light into a closed loop; and trappinglight from an external source within said closed loop until absorptionby the semiconductor.
 10. The method of claim 9 further comprising thestep of recirculating light received from an external source in saidclosed loop until absorption by the semiconductor.
 11. The method ofclaim 10 wherein said light is recirculated until all light from anexternal source is absorbed by the semiconductor.
 12. The method ofclaim 10 wherein said closed loop is a ring
 13. The method of claim 10wherein light is captured and re-circulated in the photodetector using acurved ridge-waveguide ring resonator.
 14. The method of claim 10wherein light is captured and re-circulated in the photodetector usingstraight waveguides and mirrors.
 15. The method of claim 10 whereinlight is captured and re-circulated in the photodetector using straightwaveguides and mirrors, said waveguides and mirrors redirecting lightinto said closed loop.
 16. The method of claim 10 using a photoniccrystal structure.